The present invention relates to a head feed control apparatus for a recording/reproducing apparatus using a disk-like recording medium.
Data recording tracks (to be referred to as tracks hereinafter) are concentrically or helically formed on a disk-like recording medium, such as an optical disk. A given track is traced by a head and irradiated with a laser beam to record data thereon or read it out therefrom. When data write/read operation is to be performed for successive tracks, the successive tracks can be irradiated with the laser beam by optical head tracking. However, if the tracks are to be random accessed, an access operation for moving the head to a remote track is required.
In the simplest access operation, the number of tracks crossed by the head is counted while the head is moved to adjacent tracks. When it is determined that the head has been moved to a target track on the basis of the count, the head is stopped. According to this scheme, the head can be properly moved to the target track. However, it is necessary to detect that the head crosses the tracks, thus prolonging the access time. Since high-density tracks at 1.6-.mu.m track spacings or pitch are used in recent years and the number of tracks are tens of thousands, the above access operation is unsatisfactory.
In order to eliminate this drawback, an access operation using an external scale has been recently employed. The external scale has graduations, the pitch or spacing of which is larger than the track pitch. A head feed control apparatus with the external scale is shown in FIG. 1. Disk-like recording medium 1 is rotated by spindle motor 2. A plurality of tracks 11 are concentrically formed on medium 1. Head 3 mounted on carriage 100 traces these tracks 11. Linear scale 4 is mounted on carriage 100 to move with head 3. Graduations 41 are formed on scale 4 at a predetermined pitch. Graduations 41 are read by scale detector 5 to detect an amount of displacement of head 3. Scale 4 comprises an optical or magnetic scale, or a potentiometer operated upon movement of the head 3.
For example, if pitches of tracks 11 and graduations 41 are respectively given as 2 .mu.m and 20 .mu.m, the graduation pitch of scale 4 corresponds to a 10-track displacement of head 3. Assume that the head located at track address 1000 is moved to a position at track address 1204, so that the head is moved by 204 tracks to a target track. Since 204/10=20.4, the head is moved by 20 graduations of scale 4 at high speed and then crosses four tracks one by one, thus achieving high-speed access operation.
However, a method using the scale may not be able to accurately move the head to the target track. When tracks are formed in the recording medium during its fabrication process, errors occur in the track pitch and scale graduation pitch. Disks are normally prepared by pressing with a master disk having accurate concentric tracks. All tracks are not always formed at equal intervals due to errors caused by different pressing pressures. This is also the case in the scale. For example, each pitch of graduations may not always correspond to a pitch of 10 tracks. In the access operation based on the scale, which neglects track pitch and scale graduation errors, the head cannot often be moved precisely to a target track.